1. Field of the Invention
The invention relates to a method of anisotropically etching structures preferably defined with an etching mask, particularly laterally exactly defined recesses in silicon by means of a plasma.
2. Description of the Related Art
It is known to anisotropically etch defined structures, for example trenches, crests, tongues, flexible ridges or the like having low to average selectivity, into silicon substrates.
The individual structures to be etched in are usually defined by the etching masks applied to the silicon substrate by way of so-called masking layers, for example, a photoresist layer.
In the anisotropic etching technique, it is necessary to achieve a laterally exactly defined recess in the silicon. These deeply-extending recesses must have lateral ends which are as exactly vertical as possible. The edges of the masking layers covering those silicon substrate regions that are not supposed to be etched are not underetched in order to keep the lateral precision of the structural transition from the mask into the silicon as high as possible. As a result, it is necessary to allow the etching to progress only on the bottom of the structures, but not on the already produced side walls of the structures.
To this end, it has already been proposed to use a plasma-etching method to perform etching of profiles in silicon substrates. In this method chemically reactive species and electrically-charged particles (ions) are generated in a reactive gas mixture in a reactor with the aid of an electric discharge. The positively-charged cations generated in this manner are accelerated toward the substrate by means of an electrical prestress applied to the silicon substrate, and fall virtually vertically onto the substrate surface, and promote the chemical reaction of the reactive plasma species with the silicon on the etching base.
Because of the nearly vertical fall of the cations, etching should progress correspondingly slowly toward the side walls of the structures, e.g., in the optimum case, not at all.
It is known to use non-dangerous and process-stable reactive gases based on fluorochemicals. However, in this case it is very disadvantageous that these reactive gases acting on a fluorochemical basis permit a very high etching rate and a very high selectivity, but display a markedly isotropic etching behavior.
In comparison to the silicon, the fluorine radicals generated in the plasma have such a high spontaneous reaction rate that the structure edges (lateral surfaces) are etched quickly, thus resulting in undesired underetching of the mask edges.
Moreover, it has already been proposed to cover the side walls with polymer formers which are located in the plasma at the same time during etching, and to protect the walls by means of this polymer film. Because this polymer film would also form on the etching base, a stable fall of ions should keep this film free from polymer and permit etching there. However, associated with this is the disadvantage that the polymer formers added to the plasma, which are partly formed from the fluorine carrier itself or through the splitting of fluorine radicals, or which result from purposely added, unsaturated compounds or eroded, organic mask material photoresist), occur as recombination partners with respect to the fluorine radicals. By means of this back reaction, the objective of which is a chemical equilibrium, a considerable portion of the fluorine required for etching is neutralized, while at the same time a corresponding component of the polymer formers required for side wall passivation is lost. Because of this, the etching rate that can be attained with this method is markedly reduced.
This dependence of the etching fluorine radicals on the unsaturated polymer formers in the plasma makes the etching rates and the etching profiles dependent on the free silicon substrate surface to be etched. Furthermore, it is disadvantageous that the unsaturated species present in the plasma that result in the polymer formers preferably etch certain mask materials and thus cause the selectivity, that is, the ratio of the silicon etching rate to the mask etching rate, to worsen. Furthermore, if a non-uniform side wall protection occurs, the side walls are preferably coated directly at the mask edge with polymer, and thus the side wall is better protected in this area than in the progressive etching depth of the structures.
In this instance the polymer covering of the side walls decreases rapidly at greater depths, and an underetching occurs there with the consequence that bottle-type etching profiles result.
Instead of using reactive gases based on fluorine, it has already been proposed to use reactive gases based on other halogens, particularly chlorine and bromine, which have less avidity, or reactive gases that release chlorine or bromine in plasma.
Because their radicals formed in plasma exhibit a significantly lower spontaneous reaction with silicon, and first lead to etching with simultaneous ion support, these reactive gases offer the advantage that they essentially etch only on the bottom of the structure, and not on the side walls of the structure, because the ions impact virtually vertically on the silicon substrate. The disadvantage exists, however, that these reactive Oases react in an extraordinarily sensitive manner with respect to moisture.
In this case, not only are costly transfer devices necessary for the silicon substrates in the reactor, but also the leakage rate of the entire etching system must be kept extremely low. Even the slightest occurrence of reactor moisture leads to microroughness on the bottom of the silicon etching due to local silicon oxidation, and thus to a complete breakdown in etching.
The object of the invention is to create a method of the generic type with which a high anisotropic etching of silicon substrate can be achieved with fluorochemicals with simultaneous high selectivity.